The present invention relates to a fuel-injection control device for outboard motors. More particularly, the invention relates to performance of such fuel-injection control devices during high-speed operation of a controlled engine.
Engines can use a carburetor as a means for supplying a fuel-and-air mixture to the cylinder of the engine. A carburetor positioned in the suction flow path of an engine takes advantage of the vacuum created when air is sucked in by the engine. The vacuum forces fuel to be drawn from a chamber through a jet to form a mist which mixes with air being drawn into the engine.
To compensate for the specific characteristics of the engine and the load (e.g., automobile) it drives, various different jet types can be used to provide an optimal setting. Previously, it has been impossible to have the carburetor adapt continuously to changes in driving conditions, surrounding environments, and the like. Particularly, the problem of achieving a proper setting for the air-to-fuel ratio when the engine is started is subtle and problematic.
Lately, engines employing fuel-injection have been widely used as an alternative to carburetors. A fuel-injection device can be controlled according to parameters such as the engine rotation speed, the throttle setting, the temperatures of the engine and the water used to cool the engine, and the air suction temperature. This data is processed by a computer to determine a correction value. The amount of fuel, injected directly into the suction path of the engine, can be made appropriate for the particular set of circumstances existing at any particular moment. Thus, combustion efficiency can be optimized continuously, maximizing engine output. Also, since only a minimum required quantity of fuel is injected, fuel consumption is minimized.
Referring to FIGS. 8(a) and 8(b), a conventional outboard motor M is mounted via an attachment bracket B on a vessel V. Outboard motor M pivots on the attachment bracket B permitting a trim angle .theta. to vary. The trim angle .theta. can be made appropriate for the speed of vessel V and the positioning of vessel V on the surface of water W. More particularly, trim angle .theta. can be increased when vessel speed is high and decreased when vessel speed is low. Thus, engine output can be maximized by adjusting the trim angle .theta. to maintain the propeller of outboard motor M in proper orientation with the surface of water W.
Generally, air sucked into an engine is expelled from the engine through an exhaust exit which permits the air to disperse into the surrounding atmosphere. As a result, the intake volume of air sucked into the engine can be calculated based on the throttle setting and the engine rotation speed. However, in outboard engines, the exhaust opening of the engine is underwater causing a back flow pressure of air to develop in the exhaust path. The back flow pressure in the exhaust path varies depending on the vessel speed and the trim angle. In a two-cycle outboard engine, changes in back flow pressure causes the intake volume to vary. As a result, the intake volume of air sucked into the engine cannot be accurately calculated based on the throttle setting and the engine rotation speed. When fuel injection is determined based on inaccurate calculations of intake volume the fuel consumption increases, the engine output decreases, and the exhaust gasses deteriorate.
In an attempt to reduce inaccurate calculations of intake volume, Japanese laid-open publication number 5-18287 discloses a method for detecting an exhaust pressure at an engine's exhaust port and calculating a correction value used to adjust the amount of fuel injected into the suction path of the engine. However, the high temperatures and pressures encountered at the exhaust port causes water and salt to adhere to the area surrounding the exhaust port. As a result, the exhaust pressure detecting means used in this method needs to be pressure-resistant, heat-resistant, water-resistant and salt-resistant. This increases production costs and decreases the reliability of the exhaust pressure detecting means.